In 1998, Paul F. Hoffman and Daniel P. Schrag at Harvard University put forth a chilling description of earth's climate some 650 million years ago. Their theory, dubbed snowball earth, held that between 750 million and 580 million years ago, ice repeatedly enveloped our planet, coating the seas from pole to pole and killing off early life almost completely. During the past few years, the idea has stirred up a great deal of debate. And new data published in the December issue of Geology only further throws snowball earth into question.

Lead author Martin Kennedy at the University of California, Riverside, and colleagues collected limestone and dolomite rocks from Precambrain glacial deposits in northern Namibia, central Australia and the North American Cordillera. When they analyzed these samples, they discovered that the ratio of the carbon isotope 13C to 12C was higher during the glaciation than after the ice had melted. This pattern, they say, suggests that the oceans supported a healthy ecosystem at the time¿which would be hard to do were they frozen over.

"If there was no photosynthesis or life in the ocean, the carbon isotope values would be the same as the mantle," Kennedy says. "Only the presence of life causes a difference in those values. We did not find isotopic evidence that a global ice sheet impacted overall marine productivity. We would think that if an ice sheet covered the oceans it would have had an impact on marine production or photosynthesis and we find no carbon isotopic evidence for this. The oceans just look normal.